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TPS54120: TPS54120 with 6V output

Jon Bean1
Expert 1120 points
Part Number: TPS54120

Hi

I am using the TPS54120 with a 12V input and I want a 6V output. I have calculated the feedback resistors of the switcher to give a 6.5V input to the LDO. When I power up the circuit I only see around 6V going into the LDO input. I have attached the schematic. Can someone comment if the values I have used look OK? If they are can someone suggest some things to check.

Thanks

Jon

  • 0
    TI__Mastermind 26340 points
    Hello Jon,

    I would first measure the voltage on the Vsense pin? If its below 0.8V there is likely two reasons.

    1. the device is hitting current limit
    2. the device is unstable.

    If the device is hitting current limit reduce load current and/or adjust inductor value.

    If unstable please adjust compensation to stabilize.

    Hope this helps?
  • 0 in reply to David Baba
    Expert 1120 points
    Hi David

    Yes the vsense is about 0.75V. Also the input and output voltages have huge amounts of ripple. I didnt have any load on the device, but even adding a 0.1A load didnt make any difference. So I dont thinks its a current limit issue. I guess it must not be stable.
    Thanks
    Jon
  • 0 in reply to Jon Bean1
    TI__Mastermind 26340 points
    Hello Jon,

    Are you using an input filter? If so, is it well damped? if not will need to be well damped.
    Did you follow closely the PCB layout guidelines in the datasheet? if not you may have to spin the PCB.

    Regarding stability, did you follow the app note in the link below?

    www.ti.com/.../slva503a.pdf

    You can adjust R17 to reduce the loop BW by decreasing to 10k. If this has no real effect, it is likely layout?

    Hope this helps?
  • 0 in reply to David Baba
    Expert 1120 points

    Hi David

    I reduced R17 down to 10k and the output is now stable,

    Thanks

    Jon